It is frequently desirable to produce print images from a printing liquid or ink which can be readily removed or corrected. Such printing liquids generally comprise a coloring agent or pigment and a liquid carrier or binder in which the pigment is dispersed
One of the problems with such printing liquids is that many of them tend to penetrate extremely rapidly into a bibulous or other substrate which does not have a closed surface so that the removal of the printing trace or image by mechanical means may not be possible at all or is rendered so difficult that the medium or carrier is frequently destroyed by the erasing process.
The mechanical erasure of such traces can be improved by providing the solvent with an interference factor which reduces the penetration into the substrate. Such solvents, however, create other problems which have precluded their widespread use.
Obviously it has long been desirable to utilize printing liquids for the purposes described which are water based, i.e. wherein the dispersion is a dispersion of the pigment and any binder in a vehicle consisting permanently or exclusively of water.
In German patent document (open application) DE-OS No. 29 14 913, for example, a water-based printing liquid is described which consists of an aqueous latex dispersion in which a pigment preparation in the form of a fine paste is dispersed and which contains additional substances such as preservatives. Because such liquids are not sufficiently flowable, i.e. are comparatively viscous, they cannot be utilized in so-called capillary or jet printers in which the liquid is projected upon the printing medium.
German patent document (Offenlegungsschrift) DE-OS No. 30 41 769, moreover describes an erasable printing medium which consists of a latex emulsion of a carboxylated styrene-butadiene copolymer of a predetermined viscosity and surface tension in water together with a water soluble dyestuff selected from the group which consists of triarylmethane dyestuffs soluble in the emulsion.
The problem with such printing fluids is that they tend to be extremely unstable in the presence of light and to fade even after a short time. They thus cannot be used for permanent printing purposes and the field of use is therefore extremely limited.